walker etal



Feb. 28, 1956 H. WALKER ET AL 7 2,736,080

MANUFACTURE OF THIN SHEETS OR PLATES OF HEAT-TREATED MATERIAL SUITABLEFOR THE DIELECTRICS OF CAPACITORS AND FOR OTHER PURPOSES Filed Feb. 28,1951 2 Sheets-Sheet 1 Inuenlor: l/erer-f Waller 4;"! 7' F Ste/r y JIM;MvM

Attorney;

Feb. 28, 1956 H, WALKER ET AL 2,736,080

MANUFACTURE OF THIN SHEETS 0R PLATES 0F HEAT-TREATED MATERIAL SUITABLEFOR THE DIELECTRICS OF CAPACITORS AND FOR OTHER PURPOSES Filed Feb. 28,1951 2 Sheets-Sheet 2 I nventors HM wallhw A ttorney United StatesPatent ice ,gented flifilfifl MANUFACTURE OF THIN SHEETS OR PLATES OFHEAT-TREATED MATERIAL SUITABLE FOR THE DIELECTRICS OF CAPACITORS ALJDFOR OTHER PURPOSES Herbert Walker and Tzu En Sheri, London, England,assignors to British Dielectric Research Limited, Loan, don, England, aBritish company Application February 28, 1951, Serial No. 213,107 Claimspriority, application Great Britain March 9, 1950 6 Claims.((331.25-156} In the manufacture of sheets or plates of materialsuitable for the dielectrics of capacitors the attainment of smallthickness is obviously important. It is also difficult of attainmentwith materials which in the early stages of manufacture are in the formof powder given cohesion by appropriate means. Such materials are notreadily made to flow into thin layers by moulding, pressing, or rollingunless excessive quantities of other materials are added. Theseadditions have disadvantageous results, either in later stages ofmanufacture or in the final product. Such materials if they can beformed into thin plates are usually very fragile and difiicult to handlein later stages of the manufacture. Materials to which these commentsapply are ceramic bodies, such as titanium dioxide (rutile) andtitanates of barium and strontium and of other elements. The presentinvention provides a new method of manufacturing thin sheets or platesfrom such materials in which the limitations indicated above are notpresent.

The term thin as used herein means of a thickness comparable with thatof cellulose paper.

By cellulose paper is meant paper made from cellulose pulp, such as Woodpulp or pulp made from cotton or linen rag or natural fibres, such ascotton or manilla, or mixtures of any of these materials.

The first stage of the manufacture is to produce a cellulose paperloaded with the powder to a high degree, for instance to 90% or 95% byweight, and having an appropriate thickness, for instance mils (.13mm.). This stage is preferably finished by rolling or pressing so as toobtain as dense a paper as possible. In the next stage the paper isheated so as to burn oil the other ingredients of the paper and iscontinued until the powder is brought to the temperature at which itsinters to form a sheet of strongly coherent material.

This sheet is of substantially uniform thickness. The limit of thinnessis determined mainly by the capacity to produce sufiiciently thin paper.The material in the final form is generally rigid so that any shapingrequired is preferably done while the material is still in the form ofpaper. In many cases flat sheets will be required.

It will be seen that the method imposes certain limitations on thechoice of materials to which it can be applied, particularly as to theirproperties over a large range of temperature. They should be stable andunaffected by heat at the temperature at which the other ingredients ofthe paper are driven on and should then be capable of being softened andsintered at a substantially higher temperature. Suitable materials are,for instance, ceramic materials and glass. Small quantities of flux maybe added to assist sintering.

The remainder of the paper is preferably substantially Wholly cellulosefibres and small quantities of binders which are commonly employed inthe paper industry such as starch, dextrine or gelatine. The paper maybe made in known manner, preferably beating the cellulose fibres to thehigh degree appropriate for the making of very thin paper, such ascondenser tissue. In the later stages the powdered substance is addedand beaten thoroughly with the cellulose fibres. The web of paper isthen taken up in the usual way, dried and preferably callendered. 1

As already indicated the heating takes place in two stages atsubstantially different temperatures. In the first the conditions areoxidising so that combustion of the cellulose can take place. In thelatter stage the conditions are determined by the naturepof the materialand theproperties which it is desired that the finishedproduct shallhave.

In order to give the finished plates or sheets the desired form, it maybe necessary or advantageous to hold the paper between' shaping surfacesduring the heat treatment. The attainment of a flat sheet can beassisted by ensuring uniformity of the thickness and structure of thepaper and of the heat treatment as far as possible. It may be furtherensured by placing the paper between flat plates for the heat treatment.

The following are examples of methods of manufacture in accordance withthe invention. These ex-- amples describe the manufacture of sheetmaterials of high dielectric constant, suitable for use as capacitordielectrics.

Example I The first stage is the preparation of a paper loaded withbarium titanate. This is carried out by mixing together a cellulosepulp, such as is used in paper making, with barium titanate and a smallquantity of bentonite. The ingredients are used inthe followingproportions by weight: 7 1

Cellulose pulp (solid content) 1 Bentonite l Barium titanate 98 Theslurry so obtained is spread into sheets, dried and rolled by a normalpaper making process to produce thin sheets resembling paper. The finalpressure applied may be of the order of 10 tons per'square inch.

The sheets thus obtained are arranged between flat zircon plates andfired. Four sheets for-example may be stacked between one pair of zirconplates. The best firing temperature is 1305 C. although this may bevaried from 1280-l330 C.

The total firing time is three hours including a period of /2 hourduring which the cellulose is burnt out, the temperature rising from 20C. to 500 C., afurther period of /2 hour during which the temperaturerises to and is held at the sintering temperature and a period of 2hours for cooling. l i

The proportion of ingredients given above can be varied between thefollowing limits:

- Percent Cellulose /2*-2- Bentonite /z--l Barium titanate 99.97

In all cases a strongly coherent sheet having a dielectric constantbetween 1,000 and 2,000 at room temperature is obtained. If higherproportions of cellulose are used the sheet-obtained is liable to be tooporous for use as a capacitor dielectric. If less than /2 of celluloseisused the loaded paper is too weak to handle. It is preferred to use 1%of cellulose.

For making small quantities of the sheet material described in theexample, it is possible to obtain the cellulose pulp by breaking downpaper. For example, a suitable pulp can be obtained by milling 1 gm. ofcapacitor tissue with 100 cc. of distilled water for four hours in aplane- In this example the ingredients used, in proportions by weightare:

Cellulose pulp (solid content) 1 Bentonite 1 Titanium dioxide 98 thesame process as described in Example I is used but the firingtemperature is 1450 C. The firing times are the same.

Example III In this example the ingredients used, in proportions byweight are:

Cellulose pulp (solid content) l Bentonite 1 Magnesium titanate 98 thesame process as described in-Example I is used but the firingtemperature is 1400 C. The firing times are the same.

Although the method of manufacture has been referred to particularly inconnection with the preparation of dielectric bodies for capacitors, itis to be understood that it may also be used for the preparation ofmaterials for other purposes. The applicability of the method in suchcases will be recognised by a consideration of the nature ofthematerials to be used and the form of the final product required. Itis only necessary that the material will withstand the heat necessary todestroy the cellulose and will itself sinter to form a thin plate. Themethod is applicable to the manufacture of thin sheets or plates frompowdered glass alone or mixed with dielectric ceramic materials(particularly ceramic materials with high dielectric constants),powdered metals, for example tantalum, aluminium and titanium, siliconcarbide and mixtures of carbon, silica and/ or silicates. The mixturesof carbon, silica and/ or silicates are useful in the manufacture ofelectrical resistors. In such mixtures the carbon itself does not sinterbut becomes incorporated in the ceramic material produced by thesintering of the silicon and/or silicates present. Silicon carbide isused when resistors in which resistance varies with voltage or currentare required. When materials are used which are liable to oxidise at thesintering temperature precautions are taken to exclude oxygen during thelatter part of the heat treatment.

While the upper limits of the thickness of the sheets or plates whichcan be made by the method of the invention from a single sheet of loadedpaper is controlled only by the thickness of the loaded paper sheetthere would be little advantage in using the method for the manufactureof sheets or plates thicker than about mils (.25 mm.) since such platesor sheets would probably be more easily made by known dry pressingprocesses.

In the accompanying drawing Figures 1 and 2 show two alternative formsof capacitor manufactured in accordance with the invention and Figure 3shows a process in accordance with the invention.

The capacitor shown in Figure l is a stack type made up of a number ofplates 1 of a material of the kind described in the examples. Each plateis formed on both S1dCS W1th metallised layers in accordance with normalpractice, the layer 2 on the upper surface extending over one end of theplate and onto the lower side and the layer 3 on the lower sideextending over the M 16 l fil 3 d the upper side, as shown. Metalconnecting wires 4 and 5 are soldered to the ends of the stack.

Figure 2 shows a single plate capacitor consisting of a circular plate 6of a material manufactured in accordance with one of the examplesprovided with metallised layers on its upper and lower surfaces. Thelayer 7 on the upper surface has soldered to it a connecting wire 8. Asimilar wire 9 is soldered to the lower metallised layer.

Referring to Figure 3 of the drawings an aqueous dispersion 10 ofcellulose fibres and a powdered material, formed by a heating process,is contained in a vessel through which a continuous gauze 11, of thekind used in paper making, passes round a roller 12 in a clockwisedirection. The gauze passes from the dispersion beneath the heater 13and over a tray 14 which is evacuated by a pump 15.

As the gauze rises from the surface of the dispersion 10, it picks up alayer 16 which is carried over the tray 14 and beneath the heater 13 andis dried sufiiciently to make it into a self-supporting sheet. The sheet17 is lifted continuously from the gauze 11 and passes between at leastone pair of calender rolls 18, 19. During passage through these rolls itis compressed to the re-- quired thickness and density. From the rollsthe sheet passes on to a conveyor 20 which carries it through a cut tingmachine 21 Where it is reduced to the required shapes for example discs.

In the drawing, discs 22 are shown issuing from the machine 21. Theseare placed by hand into ceramic dishes 23 which are fed successivelyinto a firing kiln: 24. In the kiln the discs are fired in the mannerdescribed in the examples. If desired, the discs, before firing, can bearranged between fiat zircon plates which are not afiected by thetemperatures reached in the firing kiln.

Both forms of capacitor are provided with a protective layer ofinsulating wax or varnish.

What we claim as our invention is:

l. A method of manufacturing a thin, strongly co-- herent electricallyinsulating sheet from a ceramic powder which comprises preparing asuspension of cellulose fibers and said powder in an aqueous liquid by abeating process, taking up a layer of said suspension of not more thanabout 10 mils thickness on a pervious support, the relative proportionsof cellulose and ceramic powder in said suspension being such that atleast by weight of the solid content of said layer consists of saidpowder, draining liquid contained in said layer through said support,drying said layer until it is self supporting, removing said layer as asheet from said support, consolidating said sheet by the application ofpressure and heating said sheet under oxidising conditions first to burnoff the cellulose and then at a higher temperature to sinter said powderand form a sheet of strongly coherent material.

2. A method of manufacturing a thin, strongly coherent electricallyinsulating sheet from a ceramic powder, selected from the groupconsisting of titanium dioxide and metallic titanate powders with orwithout small quantities of additives, which comprises preparing asuspension of cellulose fibers and said powder in an aqueous liquid by aheating process, taking up a thin layer of said suspension of not morethan about 10 mils thickness on a pervious support, the relativeproportions of cellulose and ceramic powder in said suspension beingsuch that at least 95% by Weight of the solid content of said layerconsists of said powder, draining liquid contained in said layer throughsaid support, self supporting, removing said layer as a sheet from saidsupport, consolidating said sheet by the application of pressure andheating said sheet under oxidising conditions first to burn off thecellulose and then at a higher temperature to sinter said powder andform a sheet of strongly coherent material.

3. A method of manufacturing a thin, strongly coherent electricallyinsulating sheet from a ceramic powder, selected from the groupconsisting of titanium dioxide drying said layer until it is andmetallic titanate powders with or without small quantities of additiveswhich comprises preparing a suspension of cellulose fibers, bentoniteand said powder in an aqueous liquid by a beating process, taking up alayer of said suspension of not more than about mils thickness on apervious support, the relative proportions of said ingredients of saidsuspension being such that the solid content by weight of said layer iscellulose /2-2%, bentonite /2--1% and ceramic powder 99-97%, drainingliquid contained in said layer through said support, drying said layeruntil it is self supporting, removing said layer as a sheet from saidsupport, consolidating said sheet by the application of pressure andheating said sheet under oxidising conditions first to burn off thecellulose and then at a higher temperature to sinter said powder andform a sheet of strongly coherent material.

4. A method of manufacturing a thin, strongly coherent electricallyinsulating sheet from a ceramic powder, which comprises preparing asuspension of cellulose fibers and said powder in an aqueous liquid by abeating process, taking up a layer of said suspension of not more thanabout 10 mils thickness on a pervious support, the relative proportionsof cellulose and ceramic powder in said suspension being such that atleast 95% by Weight of the solid content of said layer consists of saidpowder, draining liquid contained in said layer through said support,drying said layer until it is self supporting, removing said layer as asheet from said support, stacking a plurality of said sheets,consolidating said stacked sheets by the application of pressure andheating under oxidising conditions first to burn 01f the cellulose andthen at a higher temperature to sinter said powder and form a sheet ofstrongly coherent material.

5. A method of manufacturing a thin, strongly coherent electricallyinsulating sheet made from a ceramic powder, selected from the groupconsisting of titanium dioxide and metallic titanate powders with orwithout small quantities of additives, which comprises preparing asuspension of cellulose fibers and said powder in an aqueous liquid by abeating process, taking up a layer of said suspension of not more than10 mils thickness on a pervious support, the relative proportions ofcellulose and ceramic powder in said suspension being such that at least95% by weight of the solid content of said layer consists of saidpowder, draining liquid contained in said layer through said support,drying said layer until it is self supporting, removing said layer as asheet from said support,

stacking a plurality of said sheets, consolidating said stacked sheetsby the application of pressure and heating under oxidising conditionsfirst to burn off the cellulose and then at a higher temperature tosinter said powder and form a sheet of strongly coherent material.

6. A method of manufacturing a thin, strongly coherent electricallyinsulating sheet from a ceramic powder, selected from the groupconsisting of titanium dioxide and metallic titanate powders with orwithout small quantities of additives which comprises preparing asuspension of cellulose fibers, bentonite and said powder in an aqueousliquid by a heating process, taking up a layer of said suspension of notmore than about 10 mils thickness on a pervious support, the relativeproportions of said ingredients of said suspension being such that thesolid content of weight of said layer is cellulose /z--2%, bentonite/21% and ceramic powder 99-97%, draining liquid contained in said layerthrough said support, drying said layer until it is self supporting,removing said layer as a sheet from said support, stacking a pluralityof said sheets, consolidating said stacked sheets by the application ofpressure and heating under oxidising conditions first to burn oil? thecellulose and then at a higher temperature to sinter said powder andform a sheet of strongly coherent material.

References Cited in the file of this patent UNITED STATES PATENTS1,152,060 Sturm et a1. Aug. 31, 1915 1,447,347 Kirschbraum Mar. 6, 19232,063,102 Jones Dec. 8, 1936 2,145,151 Braunstein Ian. 24, 19392,330,106 Bernstein Sept. 21, 1943 2,391,376 Monack Dec. 18, 19452,486,410 Howatt Nov. 1, 1949 2,499,229 Rankine Feb. 28, 1950 2,539,446Lies Jan. 30, 1951 2,616,813 Klasens Nov. 4, 1952 2,643,192 Ionkers eta1 June 23, 1953 FOREIGN PATENTS 574,577 Great Britain Jan. 11, 1946OTHER REFERENCES The Bible, Exodus, ch. 5, verses 6-8, (King JamesVersion); Collins Clear Type Press, London, Eng. (1949).

1. A METHOD OF MANUFACTURING A THIN, STRONGLY COHERENT ELECTRICALLY INSULATING SHEET FROM A CERAMIC POWDER WHICH COMPRISES PREPARING A SUSPENSION OF CELLULOSE FIBERS AND SAID POWDER IS AN AQUEOUS LIQUID BY A BEATING PROCESS, TAKING UP A LAYER OF SAID SUSPENSION OF NOT MORE THAN ABOUT 10 MILS THICKNESS ON A PERVIOUS SUPPORT, THE RELATIVE PROPORTIONS OF CELLULOSE AND CERAMIC POWDER IN SAID SUSPENSION BEING SUCH THAT AT LEAST 95% BY WEIGHT OF THE SOLID CONTENT OF SAID LAYER CONSISTS OF SAID POWDER 